1. Field of The Invention
The present invention relates to an improved distillation column, particularly an improved tray. More specifically it relates to a new downcomer assembly.
2. Related Art
In the ordinary operation of a tray on distillation column the liquids flow downwardly while the gases flow upwardly. In a distillation column there are usually a plurality of plates or trays. The trays have some means of allowing vapor to flow upwardly, e.g., plurality of openings while flowing the liquid on to the tray, thereby bringing the downflowing liquid in contact with the upflowing vapors on the tray. When the liquid contacts the vapor on each tray, and mass transfer occurs, the lighter constituent(s) is concentrated in the overheads of the tower, and the heavier constituent(s) is concentrated in the lower part of the tower. The liquid moves downward from tray to tray, generally by means of downcomers.
Early downcomers are illustrated by U.S. Pat. Nos. 2,565,355; 2,582,657; 2,582,826; 2,699,929; 2,714,504 and 3,573,172, wherein each downcomer is of substantially uniform cross section. There is entrained vapor in the liquid entering the downcomers, and therefore a disengagement of vapor from the liquid in the downcomer is going to occur with a specific area of the downcomer being required to achieve this, to prevent a prematurely "flooded tower" arising. Likewise, there are entrained droplets of liquid in the vapor rising from the tray. If the upward velocity of the vapor is sufficiently low, these droplets will fall back to join the liquid on the tray from whence they came. As the upward vapor velocity increases, more and more droplets are entrained to the tray above. This liquid must then flow back through the downcomer. When the vapor velocity reaches the point that more liquid is entrained upward than the downcomer can handle, flooding occurs. Thus, in the design of a tower there is a competing factor to consider, which is the active area of the tray. The active area is mathematically defined as the cross-sectional area through which the upward flowing vapor passes. The upward velocity of the vapor is the volume of vapor divided by the "effective active area" of the tower.
The vapor handling capacity of the tower is generally proportional to the active area of the tray(s). It was found that in the downcomers that the disengagement of the vapors and liquid may be accomplished by enlarging the area of the upper portion of the downcomer and reducing the area of the lower portion of the downcomer. This in effect increased the area of the tray(s), and is illustrated for example in U.S. Pat. Nos. 2,420,075; 2,491,726; 2,591,343; 2,596,249; and 4,174,363. Hence, the improvement pursued has been to maintain the required area in the downcomer for vapor disengagement while increasing the effective active area of the tray(s).
Some unique approaches to this desirable result have been devised by the art, for example, terminating the downcomer above the liquid level of the tray below of a type like that shown in U.S. Pat. No. 4,278,621, an alternative system has been the use of a downcomer seal pan above the tray but below the liquid level and perforated bubble caps or bubble promoters of a type as shown in U.S. Pat. No. 4,510,023.
The present invention increases the effective active tray area in the column. This is achieved without loss of any of the useful downcomer volume. Actual testing showed that the improved throughput of the column was directly proportional to the increase in effective active area. It is a feature of the present invention, that by initiating vigorous contacting of the vapor and liquid as the liquid enters the tray, the general frothing action on the whole tray increases, thereby increasing the mass transfer and tray stability. It is an advantage of the present invention that the capacity (i.e., throughput of a multi-tray distillation columns) is substantially increased, i.e., the same column (same size, number of trays, feed purity, etc). It is a further advantage that the improved structure may be a retro fit, replacing substantially only part of the downcomers. These and other advantages and features will become apparent from the following.